On my walk yesterday, the world was filled with the smell of spring. The smell of flowers and barbecuing meat will be along soon enough, but early spring is dominated by the earthy fragrance after rain and the wonderful, evocative odor of new-mown grass. Lawnmowers were whining away last weekend mixing the smells of new mown grass and damp earth.
Where do these smells come from, and is there a deeper meaning to these odors than just a pleasant spring fragrance? Many of the wonderful smells of spring are of semiochemicals – chemicals produced by an organism that can affect the behavior of other organisms.
The new-mown grass smell is mostly from four closely-related six-carbon molecules that are produced by the breakdown of fatty acids. Together, they are called green leaf volatiles (GLV). As a cow or mower takes a bite of grass, these molecules are quickly produced by leaf enzymes and just as quickly evaporate. In pure form, each GLV has a pleasant, spicy fragrance, but it doesn’t remind me of the smell of new mown grass. It is the combination of the six-carbon molecules, and probably some others as well, diluted in the air, that combine to give us that wonderful smell.
It is what happens next that is quite remarkable. We are not the only creatures that can smell green leaf volatiles. We can think of green leaf volatiles as alarms, warnings and invitations. Entire communities respond to the GLV chemicals wafting through the air. For nearby plants, GLV is the equivalent of Tolkein’s Horns of Buckland sounding “Awake, Fear, Fire, Foes, Awake” and plants that smell the GLV immediately mobilize their defensive chemicals to ward off herbivores. I use “smell” here in the sense that receptors in the plant detect the chemicals, just as receptors in our nose do.
Insect herbivores that want to eat the plant can also smell the GLV. To them, it may say “someone is already eating this plant,” and they will lose interest and go elsewhere. Other insects may be attracted by GLV. For a predatory or parasitic insect looking for another insect to eat, the odor says “Hey, there are bugs eating a plant, c’mon over.” Many wasps, for example, are attracted to GLV and will attack insects that are eating the plant. So in two ways, GLV act as plant defenses: they tell nearby plants to prepare for herbivores, and they tell nearby predators and parasites to attack the herbivores.
So, while you are smelling the wonderful odors of new-mown grass, there is an entire community of plants and animals that are responding to the GLV.
And what of the earthy smell? That is another interesting story of chemical communications. Earthy fragrance comes from several sources, but the two major odors are geosmin and 2-methylisoborneol (2-MIB). Geosmin is made by an important group of soil bacteria in the genus Streptomyces. You may recognize that name as being related to the antibiotic streptymycin, and in fact many antibiotics come from soil bacteria. Geosmin and 2-MIB are also associated with poor-quality water and wine.
What might geosmin communicate to other organisms? Prof. R. Meganathan suggests that geosmin might be a cue to the availability of water. Camels
are thought to be able to smell water in an oasis from 50 miles away. Since water has no smell, it may be the geosmin that lures the camel. What does that do for the Streptomyces? Camels drinking from an oasis might carry the bacteria from one oasis to another. Of course, this is one of those Just So Stories that is hard to validate. In humans, geosmin may warn us of low-quality water or wine. The smell is often associated with fish raised in aquaculture ponds, but it can be eliminated by rinsing with lemon juice, which breaks down the geosmin.
As you take your spring walks and inhale that heady mixture of damp soil and new-mown grass, look around and think of all plants, insects and other creatures that might be sniffing the air as well.